The antibacterial defense system of the lung is the subject of the proposed research. Using animal models, the roles of physical transport of bacteria from the lungs, phagocytosis and killing of bacteria by alveolar macrophages and bacterial multiplication will be studied. Alterations in each of these parameters produced by manipulation of the host will be studied to determine at what point critical changes in lung defenses occur which predispose to pneumonia. The influence of the route of administration of the bacterial challenge will be investigated by comparing lung defenses following aerosol exposure with those following instillation of bacteria by bolus into the lungs. BIBLIOGRAPHIC REFERENCES: Francis, PB, Jay, SJ, and Johanson, WG, Jr.: The course of untreated Mycobacterium kansasii disease. Am. Rev. Resp. Dis. 111:477, 1975. Harris, GD, Johanson, WG, Jr., and Nicholson, DP: Response to chemotherapy of pulmonary infection due to mycobacterium kansasii. Am. Rev. Resp. Dis. 112:31, 1975.